The production of coatings by the electrodeposition of film-forming materials on electrically conductive substrates under the influence of an applied voltage is known. This technique is being used to an increasing extent because it reduces pollution by saving solvents and, through automation, has a rationalizing effect. Known binders for this purpose are water-dilutable, carboxyl-group-containing olefin polymer oils obtained by reacting .alpha.,.beta.-unsaturated dicarboxylic acids or their anhydrides, particularly maleic acid anhydride, optionally in admixture with .alpha.,.beta.-unsaturated monocarboxylic acids, and/or semiesters and/or semiamides of these acids with olefin polymer oils substantially free from carboxyl groups etherified, heat-reactive condensation resins of formaldehyde with phenol carboxylic acids, optionally together with polyhydroxyl compounds containing at least two hydroxyl groups and having a molecular weight of from 50 to 3000, are used as combination resins in DT-AS No. 1,929,593 (U.S. Pat. No. 3,658,795) and DT-AS No. 2,120,962 (U.S. Pat. No. 3,796,770). Crosslinking with polyalcohols is generally recommended in DT-PS No. 1,219,684 (top of column 5) (U.S. Pat. No. 3,546,184) and DT-AS No. 2,013,096 (bottom of column 2) (U.S. Pat. No. 3,766,215), although the components to be used are not defined in any detail in regard to the service properties ultimately required.
For the practical application of a maleated butadiene oil, it is necessary to use modifying resins in addition to the carboxyl-group-containing olefin polymer oil because otherwise the coatings show an inadequate crosslinking density which is unfavourably reflected in their mechanical properties. Thus, according to DT-AS No. 1,929,593 and DT-AS No. 2,120,962. hardening is improved by using the known condensation products of formaldehyde with phenol derivatives. However, with increasing concentration of so-called foreign products such as these, protection against corrosion, as determined by the salt-spray test on unbonded steel plate, generally deteriorates. In addition to the atmospheric pollution involved both in the manufacture of phenolic resins and in the stoving of these films, the pronounced sensitivity to temperature is a troublesome factor, in other words disruptive electrical charges occur at elevated bath temperatures. In addition, pure olefin polymer oils show inadequate elasticity.